1. Field of the Invention
The present invention relates to a phase difference detection type focus detection optical system for use mainly in an SLR camera.
2. Description of the Related Art
In conventional SLR cameras, a so-called TTL phase-difference detection type focus detection optical system, in which an object image formed on an expected image plane is separated into two to be reformed onto a pair of line sensors by a separator lens having at least one pair of lenses, is used as a focus detection optical system for focus state detection.
In the case where a plastic (resin-made) lens is used as the separator lens, the distance (apex-to-apex distance) between the pair of lenses of the separator lens varies with temperature variation, and this distance variation causes the distance between a pair of object images respectively formed on the pair of line sensors to vary, thus causing a focus detection error to occur. In the focus detection optical system disclosed in Patent Document 1 (Japanese Unexamined Patent Publication S60-235110) that includes a plastic separator lens, the temperature of a focus detector is detected by a temperature sensor, and data on the defocus amount is corrected in accordance with the temperature detected by the temperature sensor. However, if the detection accuracy or responsiveness of the temperature sensor is not high, a focus detection error still frequently occurs.
Patent Document 2 (Japanese Unexamined Patent Publication H02-1801) discloses a focus detector in which a separator lens is configured from a hybrid lens consisting of a plastic lens and a glass lens to reduce the apex-to-apex distance of the pair of lenses of the separator lens that is caused by variations in temperature and humidity. However, the overall length of the focus detection optical system disclosed in Patent Document 2 is great.
In the focus detector disclosed in Patent Document 3 (Japanese Unexamined Patent Publication 2008-286853), an auxiliary lens having a negative refractive power is installed in front of a separator lens to reduce the length of a focus detection optical system. More specifically, the phase-difference detection type focus detection optical system disclosed in Patent Document 3 is provided with a condenser lens positioned behind an expected image plane of a photographing lens, an auxiliary lens positioned behind the condenser lens, and a separator lens positioned behind the auxiliary lens and having at least a pair of lenses; in addition, the auxiliary lens is configured from a plastic auxiliary lens having a negative refractive power and a glass auxiliary lens having a positive refractive power, and the separator lens is configured from a plastic lens. With this optical configuration, focal point variations caused by variations of the apex-to-apex distance of the pair of lenses of the plastic separator lens and focal point variations caused by variations in refractive power of the auxiliary lens cancel each other out.
However, in the focal length detecting optical system disclosed in Patent Document 3, to cancel out focal point variations caused by variations of the apex-to-apex distance of the pair of lenses of the plastic separator lens, a plastic auxiliary lens having a large negative refractive power and a glass auxiliary lens having a large positive refractive power for countering the large negative refractive power of the plastic auxiliary lens are required; accordingly, a focal detection error due to an eccentricity between the plastic auxiliary lens and the glass auxiliary lens is apt to occur. Additionally, such an optical configuration increases the number of elements.